Pet Orthodontics

Just as one would seek to cure a painful skin condition, or treat a sore joint in their pet, one should also consider relieving pain brought on by orthodontic abnormalities. Poorly aligned teeth can lead to serious periodontal disease. In most breeds, teeth are arranged "shoulder to shoulder," in an arch. A self cleaning mechanism occurs in the arch to push food away from the teeth and gums. If the teeth are not aligned normally, food may be retained between the teeth causing inflammation and infection.
It is important to understand the head shape when determining normal bite relationships in various breeds. There are three basic head shapes for dogs. Those with long and narrow muzzles (Rough Collies, Borzoi, Doberman, Greyhound, Saluki); those with a short and wide muzzles (Bulldog, Pug, Pekingese, Boxer, Boston Terrier, Shih Tzu); and dogs with medium length and width muzzles (Labrador Retriever, German Shepherd Dog, most Spaniels, Terriers, and Hounds). Cats also have different head shapes which vary from the short muzzled Persian to the longer muzzled Siamese.

Normal scissors bite, notice the midline of upper and lower jaws are aligned Normally, the lower canine should intersect the upper lateral incisor and upper canine

The normal bite of dogs and cats with medium or long length and width muzzles is called a scissors bite. The upper incisors are located in front of the lower incisors when the mouth is closed, and there is a smooth curve from canine to canine without misplaced or rotated teeth. The lower canines should lie exactly between the upper lateral incisors and upper canines, yet touching neither. Premolar crown tips should point to a space between the crowns of the opposing premolars. In dogs that have a short, wide muzzle, a reverse scissors bite is considered normal where the lower incisors are in front of the upper incisors. The lower canines and premolars will also be shifted forward. While the reverse scissors bite is an acceptable breed standard, at times the upper incisors cause trauma and pain to lower jaw tissues.

Genetic or Not

Normal interdigitation of premolars

Occlusion is controlled by genetics, nutrition, environment, and by mechanical forces generated by the interlock of the upper and lower teeth. Some abnormal bites (malocclusions) have been proven to be genetically influenced, such as severe over or under bites, and wry bites. Other bite abnormalities are known to be acquired (non genetic). Acquired malocclusions can result from tug of war games played with towels or ropes which move teeth into abnormal position. Traumatic birthing can also responsible for some acquired abnormalities.

This is a genetic malocclusion because the premolars do not line up in a pinking shear arrangement

To help define whether the malocclusion is genetic in origin, interdigitation of the premolars is studied. In breeds that have medium and long muzzles, the premolars should meet in a saw-toothed fashion. For example, the tip of the lower third premolar should be positioned equally between the crowns of the upper third and fourth premolars. If the tip of one premolar points to the tip of another premolar, there may be a genetically induced malocclusion. This only holds true in those breeds that do not have shortened muzzles.
Some genetic bite problems do not show up in each litter because they are recessively passed on. The goal of selective breeding is to mate one animal to another that has superior occlusion.

Retained Deciduous Teeth

Retained upper canine tooth

Normally the deciduous tooth's root is resorbed, making room for an adult tooth. Should this fail, the adult tooth may deviate from it's normal position, producing malocclusion. The resulting double set of teeth overcrowds the dental arch, causing food to become trapped between the teeth, leading to early periodontal disease. A double set of roots may also prevent normal development of the socket, and erode periodontal support around the adult tooth, resulting in early tooth loss. A retained deciduous tooth should be extracted as soon as an adult tooth is noted in the same area as the baby tooth. If extraction is performed early, the abnormally positioned adult tooth usually moves to it's normal location.
A procedure performed by some breeders is to trim or cut deciduous teeth in hopes that they will be shed early preventing orthodontic problems. By cutting the tooth in half, pulp is exposed to oral bacteria causing infection, pain, and tooth loss. Unfortunately, the remaining infected root can interfere with the emerging adult tooth, which may not come in normally.

Dental Interlock

Jaws do not grow at equal rates. If deciduous teeth erupt during an accelerated growth phase of one jaw, an interlocking of both sets of primary teeth can maintain the abnormal bite relationship. Even genetically normal dogs can occasionally develop abnormal bites due to the interlock of primary teeth. If an under bite is noted before the permanent teeth erupt, treatment may be helpful. Removal of primary teeth from the shorter jaw that interfere with forward growth, if performed by ten weeks of age may allow the upper jaw to lengthen unimpeded. This procedure called interceptive orthodontics will correct about 50% of minor jaw length malocclusions, by the time permanent teeth erupt. Extraction does not stimulate jaw growth, it only removes a mechanical barrier to genetic control of the growth process.
Teeth that are crowded, rotated, or tilted at abnormal angles can result in:

	Early onset and increased severity of oral infection
	Damage to the soft tissues of the mouth, due to sharp teeth that penetrate the unprotected gum and mouth tissues. In addition, the lower canines can erode through the hard palate, causing food to enter the nasal cavity
	Excessive wear when abnormally aligned teeth grind against each other. Such abrasion frequently wears through enamel, causing a weakened tooth to fracture and expose the root canal system.
	Pain in joints of the jaw

Breeders, show judges, veterinarians, and others who wish to describe specific dental conditions in dogs and cats need to use proper orthodontic terms. Over bite, open bite, over jet, level bite, overshot, under bite, anterior cross bite, posterior cross bite, wry bite, and base narrow canines are orthodontic terms that are at times confusing.

Missing or Extra Teeth

Missing lower fourth premolar 8 upper baby incisor teeth in boxer puppy (normally should have only 6)

Dogs and cats may be born without the proper number of teeth. Extra (supernumerary) teeth can cause periodontal disease from over crowding. The American Kennel Club sets standards concerning minimum number of teeth accepted for each breed to be considered for show. Dental x-rays can be taken as early as ten weeks of age to evaluate if the correct number of teeth are present. X-rays are recommended as a part of the prepurchase examination in certain breeds. Missing teeth (hypodontia) usually occurs in the premolar area, but any tooth in the mouth may not erupt. Missing or extra teeth are considered genetic faults. Collies and Doberman Pinchers are most commonly affected. Sometimes the missing tooth is trapped below the gum line. A dental x-ray can be taken to determine if an unerupted tooth is present.

Misdirected Canine Syndrome

Rostrally deviated upper canine tooth treated by crown reduction or elastic to pull back tooth into normal occlusion

Misdirected canine syndrome is a bite abnormality, in which retention of the deciduous tooth tilts the erupting permanent canine tooth into abnormal location. The opposing canines may not have room to occlude properly, resulting in abnormal wear, periodontal disease, or early tooth loss.

Occlusion

The way which teeth align with each other is termed occlusion. Normal occlusion in most medium and long muzzled breeds consists of the upper (maxillary) incisors that just overlap the lower (mandibular) incisors (scissors bite). The lower canine should be located equidistant between the corner (lateral) incisor and the upper canine tooth. Premolar tips of the lower jaw should point between the spaces of the upper jaw teeth.

Malocclusion

Open bite - incisors do not meet

Malocclusion refers to an abnormal tooth alignment. Over bite (parrot mouth, over shot, class two, over jet, mandibular brachygnathism) occurs when the lower jaw is shorter than the upper. There may be a gap between the upper and lower incisors when the mouth is closed. The upper premolars are displaced at least twenty-five percent toward the front, compared to the lower premolars. An over bite malocclusion is never considered normal in any breed and is a genetic fault. The most commonly affected breeds are those with elongated muzzles (Collies, Shelties, Dachshunds, and Russian Wolfhounds).

Underbite, undershot prognathic skelatal malocclusion

An under bite (under shot, reverse scissors bite, prognathism, class 3) occurs when the lower teeth protrude in front of the upper jaw teeth. Some short muzzled breeds (Boxers, English Bull Dogs, Shih-Tzus, and Lhasa Apsos) normally have an under bite, but when it occurs in medium muzzled breeds it is abnormal. When the upper and lower incisor teeth meet each other edge to edge, the occlusion is considered an even or level bite. Constant contact between upper and lower incisors can cause uneven wear, periodontal disease, and early tooth loss. Level bite is considered normal in some breeds, although it is actually an expression of under bite.

Level Bite

Anterior Crossbite

Wry Bite

 

Base Narrow Canines

Anterior cross bite occurs when canine and premolar teeth on both sides of the mouth occlude normally but one or more of the lower incisors are positioned in front of the upper incisors. This condition can be caused by tug-of-war games, retained primary teeth, or impacted roots. Anterior cross bite is a common malocclusion. It is not considered a genetic or inherited defect. Posterior cross bite occurs when one or more of the premolar lower jaw teeth overlap the upper jaw teeth. This is a rare condition that occurs in the longer-nosed dog breeds. A wry mouth or wry bite occurs when one side of the jaw grows more than the other. Wry bites show as triangular defects in the incisor area. Some of the incisors will meet their opposing counterparts, while others will not. Wry bite is a severe inherited defect. Base canines occur when the lower canine teeth protrude inward often producing damage to the upper palate. This condition is either due to retained deciduous teeth, or an overly narrow mandible. Base narrow canines may be corrected through orthodontic devices that push the teeth into normal occlusion. An open bite occurs when some incisors are displaced vertically and do not touch. Often, the tongue will protrude. Rotated teeth, commonly affecting the upper third premolar, occurs mostly in short muzzled breeds. 

Inclined Plane

Selective breeding has created undersized mouths that cannot accommodate forty two teeth in normal alignment. The rotated tooth root closest to the palate is prone to periodontal disease. Strict tooth brushing may be helpful in saving a rotated tooth, but, frequently, the tooth cannot be saved.

Orthodontic Care

Many abnormal bites can be corrected. Orthodontic care should be performed by veterinarians familiar with tooth movement principles. Breeders at times use rubber bands which may move teeth, but compromise the gum tissue around teeth leading to periodontal disease, pain, and early tooth loss. Orthodontic care should be reserved to ease pet discomfort, by realigning teeth. Tooth movement is accomplished by employing brackets, acrylic retainers, springs, and elastics.